It is known that the grip performance of a pneumatic tire is greatly affected by tire temperature, and sufficient grip performance is not obtained in the low-temperature state. In particular, racing tires for running on circuits must have the characteristic that the rubber composition that constitutes the treads reaches the high-temperature state as quickly as possible after travel begins. Therefore, a large quantity of carbon black is compounded in rubber compositions for use in treads. However, rubber hardness, elastic modulus and rubber strength of rubber compositions in which a large quantity of carbon black is compounded will easily decrease in the high-temperature state. For this reason, if high-speed travel continues for a long duration, wear resistance decreases, wear resistance of the tread surface will be negatively affected, dry grip performance gradually decreases due to heat sag, and, depending on the case, a blowout may occur.
Japanese Unexamined Patent Application Publication No. 2007-246625 proposes suppressing the decrease in rigidity due to heat generated while traveling and improving dry grip performance and heat sag resistance by compounding carbon black of small particle size in styrene-butadiene rubber having a high glass transition temperature as a rubber composition for use in tire treads. However, the required performance of racing tires demanded by users has further increased, and a rubber composition for use in tire treads that has excellent dry grip performance, has further improved rubber hardness, elastic modulus and rubber strength in the high-temperature state, and can maintain excellent dry grip performance for a longer time is demanded.